Food products which are packaged in so-called "cook-in" films are well known in the art. Cook-in films, either in the form of tubular casings or bags, are used for packaging a food product such as meat wherein the food product is enclosed in the film and then processed, e.g. cooked, in situ usually by immersion in a hot water bath. Typical food products packaged and processed in this fashion include, among others, hams, turkey hams, fish and poultry rolls.
Food products packaged and processed in this fashion are often sold to the retail consumer market still encased in the film. As an alternative, the processor has the option to remove the film after cooking and repackage the cooked food product either whole or sliced, for retail sale. In either case, a very desirable feature of the food product package is that the cook-in film suppresses the formation of "cook-out", also referred to as "purge". The terms "cook-out" and "purge" refer to the liquid which tends to exude from a food product during cooking. This liquid exudate generally comprises water, water-fat emulsion, broth or other food juices. Cook-out is objectionable for several reasons. For example, it will cause a layer of liquid to form between the film and food, and also collect as pools in any voids between the film and the surface of the food product. In cases where the cook-in film is not removed for retail sale, these layers or pockets of exudate liquid detract from the appearance of the package and may deter consumer purchase. Also, this accumulation of liquid adversely affects the preservability of the cooked, packaged food.
The exudate liquid further represents an undesirable weight loss. This is especially true in cases where the cook-in film is removed for slicing and/or repackaging, in that the liquid is not reincorporated into the repackaged food product.
To a large extent, the quantity of cook-out or purge is dependent upon the ability of the surface of the food product to wet the food contacting surface of the packaging film. If the film surface is wetted by the food product, the film will adhere to the food product surface during cooking and this adherence will prevent cook-out.
"Wetting" is defined for purposes of the present invention as an affinity between the film surface and the food. One indication of this affinity is the wetting tension of the film surface as measured by ASTM D2578-67.
Using meat emulsion as an example, adherence of a film to the meat product surface will increase as the wetting tension of the film surface increases. However, if the adhesion is too high, the meat surface is disrupted by chunks of the cooked meat adhering to the cook-in film when the film is removed. In practice, the adhesion must be controlled to provide the adhesion necessary to prevent cook-out while keeping the adhesion below a level which causes chunks of cooked meat to pull off when the cook-in film is removed.
A further advantage of having the cook-in film adhere to the food is that it provides the package with a satisfactory outer appearance. This is especially desirable where the cook-in film is not removed prior to sale at retail.
Accordingly, a cooked food product package including a cook-in film encasing a food product cooked in situ within the film wherein the food product wets the film so the film tightly adheres to the cooked food surface not only improves the package appearance but also extends shelf life and reduces weight loss attributed to the liquid cook-out.
Other desirable characteristics for cook-in films include high tensile strength and puncture resistance at typical cooking temperatures (68.degree.-82.degree. C.) and, for certain end uses, the film should have a low oxygen permeability and be heat shrinkable. These desirable cook-in film characteristics are preferably obtained by constructing a laminate film. For example, a meat adhering cook-in film as disclosed in U.S. Pat. No. 4,784,863 is a three layer film wherein an oxygen barrier layer comprising a vinylidene chloride-methyl acrylate copolymer (MA-VDC) is disposed between inner and outer layers. The outer layer is composed of a material, or a blend of materials, able to withstand the abuse and abrasion of handling the package. Suitable outer layers as disclosed in the '863 Patent include linear low density polyethylene (LLDPE), very low density polyethylene (VLDPE), ethylene-vinyl acetate (EVA) or blends of these materials. The inner layer, which includes the film surface in direct contact with the meat product, is an EVA having a vinyl acetate (VA) content of between about 3% and about 18%.
U.S. Pat. No. 4,888,223 (the disclosure of which is incorporated by reference) discloses a cook-in film of two or more layers. In a three layer embodiment, the outer layer is composed of nylon. Nylon provides the required barrier properties and is abuse and abrasion resistant. The innermost layer is a polyethylene. An intermediate adhesive layer bonds the outer nylon layer and inner polyethylene layers together.
It also is known in the art to subject the cook-in film to various treatments to improve its food adhering characteristics. For example, U.S. Pat. No. 4,411,919 discloses that by subjecting the food adhering surface of a polymeric olefin film to an energetic radiation in the presence of oxygen, the surface is oxidized to render the surface characteristics of the film more compatible with the surface characteristics of the food product. The '919 Patent discloses that suitable energetic radiation treatments include corona discharge, flame, plasma, ultraviolet and electron beam radiation.
In the '863 Patent mentioned above, the food adhering property of the inner EVA layer of the cook-in film is improved by dispersing starch particles across the food adhering film surface and then irradiating the film. In the above mentioned '223 Patent the food adhering property of the polyethylene film inner layer is increased by subjecting the food contacting surface to a corona treatment.
The film inner surface of Patent '863--irradiated starch containing EVA--has acceptable cook-out (purge) as measured by good meat adhesion and little "fat out" (i.e. accumulation of high fat content material in concentrated form between the meat outer surface and the film inner surface) for most meats including premium grade boiled ham having less than about 10% fat and usually less than about 5% high collagen meat protein of the total available meat protein. Unfortunately when the meat to be cooked insitu is a commodity style boiled ham having a product composition of more than about 10% fat and usually more than about 5% high collagen meat protein of the total available meat portion, the irradiated dispersed starch particle-containing EVA inner layer-to-meat adhesion is only marginally satisfactory. Improved cook-out (purge) as measured by meat adhesion for commodity style boiled ham has been realized with the film surface of U.S. Pat. No. 5,051,266, comprising a blend of between about 30% and about 75% of the aforementioned EVA and between about 25% and about 70% of an unneutralized acid copolymer of an alpha-olefin. This compound has the formula RHC=CH.sub.2 where R is H or C.sub.1 to C.sub.8 alkyl and an alpha, beta-ethylenically unsaturated carboxylic acid, as for example ethylene acrylic acid (EAA). The film surface is irradiated at dosage of at least about 2 MR.
The Patent '266 type film surface--an irradiated EVA-EAA blend--provides improved cook-out (purge), but based on commercial use, even further improvement i.e. lower cook-out (purge) is desirable for high collagen, high fat type meats.
There is a need for an improved method for corona treating the inside surface of a flexible thermoplastic tubular film. In the prior art method as for example described in the aforementioned U.S. Pat. No. 4,888,223, a tube is inflated with gas in an amount at least sufficient to prevent the contact of internal surface areas of the tube. This transverse space corresponds to the distance between the opposing corona discharge electrodes. Roller-type electrodes are located in transversely positioned pairs with each pair longitudinally spaced from each adjacent pair of electrodes, with one roll member being a discharge electrode and the other roll member being a grounded electrode.
Since the prior art roller electrodes support the longitudinally moving gas inflated tube, there is no air gap between the tube outer surface and the supporting roller electrode surface, and it is not possible to simultaneously apply a significant corona discharge to the tube outer and inner surfaces, only the latter. Such simultaneous treatment may be desirable to increase the wetting tension of the inner surface for improved meat adhesion during cook-in, and also increase the wetting tension of the outer surface for printing thereon.
Also in the prior art system for corona treatment of inside surfaces of flexible thermoplastic tubular films, each roller electrode requires a slip ring to transfer electricity from a stationary member to the rotating electrode, and these slip rings have a high wear rate. Finally the prior art corona treatment system requires power driven nip rolls for longitudinal movement of the gas-supported tube through the corona-discharge region.